1. Technical Field
The present invention relates to a test apparatus and a test method for testing a device under test. More specifically, the present invention relates to a test apparatus and a test method for testing an degree of eye opening of a signal which is output from the device under test.
2. Related Art
A test of a quality of output waveforms is one item in tests for a device under test such as a semiconductor chip. For example, such test may be performed by causing the device under test to output a pseudo random pattern to measure the degree of eye opening of an eye diagram of an output signal. In order to measure the degree of eye opening in a timing direction, jitter of the edge of the output signal is measured, meanwhile a mask test is performed in order to measure the degrees of eye opening both in the direction of timing and in the direction of voltage level.
The mask test is to measure whether all waveform data of the output signal exist outside an eye mask, supposing that the eye mask according to the quality requirement is located at the center of the eye diagram. For example, the eye mask is often defined as the standard of an interface. Therefore, when mask tests are applied to a mass production test, it is preferred that they are performed with multichannel in parallel and performed in a short period of time.
Conventionally, the application for performing mask tests is incorporated in a separate measuring device. For example, some sampling oscilloscopes may include application software for the mask test. In addition, such as a time interval analyzer may be provided with the application software for the mask rest. Here, the following document discloses prior art for measuring the eye opening.    Patent document 1: Japanese Patent Application Publication No. 2000-295298
However, it is difficult to perform mask tests for the mass production test by an apparatus in which the separate measuring device, such as the sampling oscilloscope and the time interval analyzer is provided with the software for the mask test. For example, when measuring with the multichannel, many oscilloscopes may be required, therefore, it is difficult to implement the measuring device with the multichannel in a high density.
Moreover, a process to store the measured signal waveforms in a memory and to determine whether all the signal waveform data exist outside the eye mask is performed by such as a processor in general, however, the processor may not process in less time. In addition, since the signal waveforms are measured by equivalent sampling, it takes a long time to measure the signal waveforms. Moreover, when the signal waveforms are under-sampled, the dead band in the measurement occurs. Furthermore, it is required that the phase of a strobe signal that defines a sampling timing is gradually changed and scanned.